In many processes used to fabricate devices, various kinds of photoetching processes are used. Such processes are highly advantageous in many instances because often etching can be confined to certain designated areas. Even more attractive are photoetching processes where the rate of etching is dependent on light intensity. Here, the possibility of etching various geometrical features directly on a surface (such as a lens on the surface of a light emitting diode or photodetector) becomes evident.
In addition, in many device fabrication procedures, large arrays of devices are fabricated simultaneously. Typical examples are in semiconducting processing such as the fabrication of semiconductor lasers, light emitting diodes, etc. Here, a semiconductor wafer is processed with many (generally more than 20 but often several hundred) individual devices on the wafer. A mask used for a photoetching process generally must have a repetitive pattern to reflect the large number of devices on the wafer. Also, the masks often have rather minute detail because of the small size of the individual devices and the detailed features of each individual device. It is also convenient that the mask structure be stable, relatively independent of radiation energy (wavelength of the light used in the photoetching process) and easily made.
A particular example of the above is a photoetching process for making integral lenses on light emitting diodes (LEDs). LED devices are fabricated on a semiconductor wafer, many such devices on each wafer. These devices are equally spaced on the wafer. Each LED device requires a lens of precise shape to collect radiation from the LED and direct such radiation in a particular direction (for example, coupling the radiation into an optical fiber). To obtain a lens of specific geometry using a photoetching technique, the spatial variation of the photoetching radiation must be very precise for each LED on the wafer. Particularly important for such a process is a mask structure which is easily and precisely made, is stable both as to dimensions and light transmission, and is relatively independent of the wavelength of the photoetching radiation used. This last requirement permits use of relatively broad-band radiation sources (such as a tungsten light source) for the photoetching process.